Add upgrade docs for 4.6.x to 6.1.x

Author: runeflobakk

README.md

@@ -39,3 +39,28 @@ And depend directly on the library artifact (without any specified version, sinc
     </dependency>
 ...
 ```
+
+
+## Upgrading
+
+> [!IMPORTANT]
+> Clients are always strongly encouraged to upgrade to the [latest](/releases/latest) version.
+
+The _minimal_ version which is currently supported is [v6.1](/releases/6.1). Any earlier release is expected to stop working after January 7th 2025 due to Posten signering transitioning from SEIDv1 to the SEIDv2 certificate standard in the API. (Note: SEIDv2 used for clients' certificates is not affected, and has been supported for several years already. No action is required for client certificates.)
+
+
+
+
+### Changes v4.6.x ➡️ v6.1.x
+
+
+- change all references to `Document.FileType` to `DocumentType`
+- multiple documents in a signature job is now supported, and both [PortalJob.builder(..)](https://javadoc.io/static/no.digipost.signature/signature-api-client-java/6.1.1/no/digipost/signature/client/portal/PortalJob.html#builder(java.lang.String,java.util.List,java.util.List)) and [DirectJob.builder(..)](https://javadoc.io/static/no.digipost.signature/signature-api-client-java/6.1.1/no/digipost/signature/client/direct/DirectJob.html#builder(java.lang.String,java.util.List,java.util.List,no.digipost.signature.client.direct.WithExitUrls)) methods are changed according to this:
+    - the **title of the job** is the first parameter (this was previously set on the document itself)
+    - the second and third parameter are either **lists of documents and signers**, or a single document and a single signer
+    - for direct jobs, **exit-URLs** are provided as the fourth parameter
+    - the optional description for the job (text with more details displayed for the signer) is set on the builder instance: [.withDescription(String)](https://javadoc.io/static/no.digipost.signature/signature-api-client-java/6.1.1/no/digipost/signature/client/direct/DirectJob.Builder.html#withDescription(java.lang.String))
+- availability for portal jobs:
+    - The [PortalJob.Builder.availableFor(..)](https://javadoc.io/static/no.digipost.signature/signature-api-client-java/6.1.1/no/digipost/signature/client/portal/PortalJob.Builder.html#availableFor(java.time.Duration)) now accepts a [Duration](https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/time/Duration.html) instead of separate `long` and `TimeUnit` arguments.
+- response from `DirectClient.create(DirectJob)`
+    - In the case of jobs with a single signer, it was previously possible to get the signer's redirect URL from `DirectJobResponse.getSingleRedirectUrl()`. This is now changed to `DirectJobResponse.getSingleSigner()` and you can [get the redirect URL](https://javadoc.io/static/no.digipost.signature/signature-api-client-java/6.1.1/no/digipost/signature/client/direct/DirectSignerResponse.html#getRedirectUrl()) from the returned single `DirectSigner` instance. The URL is represented as a `java.net.URI` instead of previously being a `String`.